Camera surveillance has become more and more common. Images captured by surveillance cameras are for example used as evidence in police investigations and legal trials for the purpose of identification of objects, such as the face of a suspect or the numbers on a license plate.
In order for the object identification to be valid as legal evidence, the resolution of the object in the image must be high enough, i.e. the object must cover a certain amount of pixels in the image. This is usually expressed in terms of a pixel density requirement specifying a pixel density (e.g. measured in pixels/m or pixels/m2) which enables identification of the object from the image. For example, police authorities may recommend using a certain pixel density for identity recognition of a face. The recommended pixel density, i.e. the pixel density requirement, may vary between different types of objects. For example, 250 pixels/m may be recommended for face recognition, whereas 100 pixels/m may be recommended for license plate recognition.
In surveillance installations where object detection is important it is therefore crucial to set up an optimal focal length, achieved by setting the level of zoom. The camera should ideally have an as large field of view as possible at the same time as it should capture objects according to the pixel density requirement at a certain distance from the camera.
US 2013/0155295 A1 discloses a method for maintaining a minimum pixel density across an object of interest. In brief, the disclosed method determines a current pixel density of an object in a frame of video, and compares the current pixel density to a minimum pixel density. Based on the comparison, a zoom feature of the camera may be adjusted so as to adjust the current pixel density with respect to the minimum pixel density.
However, the pixel density requirements as for example recommended by police authorities are only valid under optimal light conditions. When scene conditions change, for example when the light level changes, the recommended pixel density requirement may not be enough to ensure a legally valid object identification—a higher pixel density may well be required. It is thus desirable to be able to determine a need for a change in a pixel density requirement due to changing light conditions.